The invention relates to dual band antennas including but not limited to dual band flat plate array antennas and dual band array feeds for reflector antennas.
Domestic satellite communication antennas are widely used to receive signals such as television broadcasts rather than to transmit as well as receive. However, demand for interactive services such as interactive television and use by small office/home office users has led to the requirement for domestic two-way satellite communication to be provided.
This is possible by using two antennas, one for an up-link or transmission signal and one for a down-link or reception signal. However, this increases the cost of the equipment needed by a subscriber and also increases installation, transport and maintenance costs. The space required for the antennas is also greater and this is a particular problem for domestic applications where space is at a premium.
The up-link and down-link signals are provided at different frequency bands in order that they are readily distinguishable and do not interfere. Antennas which provide two frequency bands are referred to as dual band antennas and a number of different types of dual band antennas are known. However, these suffer from a number of drawbacks when considering subscriber satellite communication systems.
For example, frequency selective surfaces can be used to provide dual bands as in earth station antennas. FIG. 1 is a schematic diagram showing use of a frequency selective surface 133. Signals from a transmitter 131 reflect from the frequency selective surface 133 and onto a reflector 130. However, signals received at a different frequency and reflected from reflector 130 towards the frequency selective surface pass through that surface 131 towards a receiver 132. That is, the frequency selective surface is arranged to reflect signals of a certain frequency range and transmit others. In this way dual band communication using only one main reflector 130 is possible. However, this type of system is difficult and expensive to install because four components, the transmitter 131, receiver 132, frequency selective surface 133 and reflector 130, must all be correctly aligned. This is difficult to achieve at low cost. Another problem is that cabling must be provided to the transmitter and receiver separately because these have different locations. This also increases installation costs.
Another approach has been to provide a dual band feed for a reflector antenna. For example, this type of system is described in U.S. Pat. No. 4,740,795, Seavey. Two coaxial waveguides are used for the respective two frequency bands and in order that the beamwidth of each beam is similar (and arranged to cover the reflector surface) these waveguides are of different diameter. In order to accommodate this arrangement the design is complex and expensive. In addition, dual band feed systems such as that described in Seavey are not suitable for monopulse alignment methods or for distributed power amplification.
Monopulse alignment methods enable an antenna to be accurately aligned with respect to a satellite and this is particularly important in subscriber satellite communication applications where there is typically little room for alignment error and where costs for an operator to align an antenna are high. Distributed power amplification is advantageous because high power transmit amplifiers are not readily available at millimetric frequencies. In dual band feed systems such as the Seavey system, distributed power amplification is not possible because there is only one transmit antenna element.
U.S. Pat. No. 4,141,012, Hockham et al. describes a dual band waveguide radiating element for an antenna. Using this element an array antenna which operates at two frequencies can be provided. The waveguide element is excited by probe structures entering the guide perpendicular to the plane of the array face. This has significant cost and size implications because the antenna is not a xe2x80x9cflat-platexe2x80x9d. Also, in terms of the number of elements being fed the approach described in U.S. Pat. No. 4,141,012 is inefficient.
A general rule in antenna design is that, in order to xe2x80x9cfocusxe2x80x9d the available energy to be transmitted into a narrow beam, a relatively large xe2x80x9caperturexe2x80x9d is necessary. The aperture may be provided by a broadside array, a longitudinal array, an actual radiating aperture such as a horn, or by a reflector antenna which, in a receive mode, receives a collimated beam of energy and focuses the energy into a converging beam directed toward a feed antenna, or which, in a transmit mode, focuses the diverging energy from a feed antenna into a collimated beam.
Those skilled in the art know that antennas are reciprocal devices, in which the transmitting and receiving characteristics are equivalent. Generally, antenna operation is referred to in terms of either transmission or reception, with the other mode being understood therefrom.
A particular problem with respect to feeds for reflector antennas is that manufacturing costs are relatively high because many parts are required and the overall structure is complex. For example, the structure described in U.S. Pat. No. 4,740,795, Seavey, above is particularly complex and expensive. Often special connectors are required and complex shielding is necessary to prevent leak of electromagnetic radiation. Also, because many different parts are used, each of these has to be tested individually which increases manufacturing time and makes maintenance and repair difficult. These factors increase the cost of feeds which is particularly disadvantageous for domestic systems intended for mass production.
It is accordingly an object of the present invention to provide a dual band antenna which overcomes or at least mitigates one or more of the problems noted above.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.
According to a first aspect of the present invention there is provided a dual band flat plate antenna comprising:
(i) a first flat plate array of first single band antenna elements operable at a first frequency band;
(ii) a second flat plate array of second single band antenna elements operable at a second frequency band different from the first frequency band, said second flat play array being positioned over said first flat plate array such that said second single band antenna elements overlie regions of said first flat plate array between said first single band antenna elements; and wherein
(iii) one or more regions in said second flat plate array are removed such that in use, said first single band antenna elements are operable through said second flat plate array.
This has the advantage that a compact, low cost antenna array is provided that operates at two frequency bands and which has a flat-plate form. The flat-plate form enables the antenna to be easily installed and for electronics associated with the antenna to be mounted on the back of the flat-plate.
According to a second aspect of the present invention there is provided a dual band flat-plate array feed for a reflector antenna said array feed comprising:
(i) a first flat plate array of first single band antenna elements arranged to operate at a first frequency band;
(ii) a second flat plate array of second single band antenna elements arranged to operate at a second frequency band, said second flat play array being positioned over said first flat plate array such that said second single band antenna elements overlie regions of said first flat plate array between said first single band antenna elements; and wherein
(iii) one or more regions in said second flat plate array are removed such that in use, said first single band antenna elements are operable through said second flat plate array.
This has the advantage that by superimposing the first and second flat plate arrays a compact and low cost feed is provided that operates at two frequency bands. Monopulse alignment methods can be used to correctly align the antenna during installation and this reduces installation costs. Also, distributed power amplification can be used. In this way a low cost, dual band, compact, array feed is formed for a reflector antenna.
According to another aspect of the present invention there is provided a reflector antenna comprising a dual band flat-plate array feed said flat-plate array feed comprising:
(i) a first flat plate array of first single band antenna elements arranged to operate at a first frequency band;
(ii) a second flat plate array of second single band antenna elements arranged to operate at a second frequency band, said second flat play array being positioned over said first flat plate array such that said second single band antenna elements overlie regions of said first flat plate array between said first single band antenna elements; and wherein
(iii) one or more regions in said second flat plate array are removed such that in use, said first single band antenna elements are operable through said second flat plate array.
In this way a low cost, dual band, compact, reflector antenna is formed that can be used for subscriber satellite communication systems such as satellite television.
The present invention also encompasses a method of operating a dual band flat plate array antenna as described above said method comprising the steps of:
(i) transmitting information input by a user to a satellite using said second single band antenna elements; and
(ii) receiving signals from said satellite using said first single band antenna elements, on the basis of said transmitted information.
This provides the advantage that using the dual band flat plate array antenna a user is able to communicate with a satellite, for example, in a satellite television system. The user is then able to access communications systems to which the satellite is linked, such as the internet.
The present invention also provides a method of operating a reflector antenna as described above said method comprising the steps of:
(i) transmitting information input by a user to a satellite using said second single band antenna elements; and
(ii) receiving signals from said satellite using said first single band antenna elements, on the basis of said transmitted information.